Crystals of KTiOPO.sub.4 and its analogs are considered highly useful because of their nonlinear optical properties. U.S. Pat. No. 3,949,323 teaches the use of flaw-free crystals in the nonlinear optical and electro-optical applications. In addition, Sil'vestrova, et al. ("Piezoelectric, elastic, and dielectric properties of RbTiOPO.sub.4 ", Soy. Phys. Crystallogr. 35, 140-141 (1990) reported that these crystals should be useful in piezoelectric applications. Whereas the growth of large inclusion-free single crystal of KTiOPO.sub.4, KTiOAsO.sub.4, RbTiOPO.sub.4 and RbTiOAsO.sub.4 were successfully demonstrated by Cheng, et. al., "Crystal Growth of KTiOPO.sub.4 Isomorphs from Tungstate and Molybdate Fluxes", J. of Crystal, 110, 697-703 (1991), the growth of large (e.g., about 5.times.5.times.5 mm.sup.3 or greater) single crystals of CsTiOAsO.sub.4 was unsuccessful using similar flux growth procedures. Nevertheless, due to its significantly lower ionic conductivity and its ability to efficiently frequency double the 1.3 .mu.m line of the Nd:YAG laser, CsTiOAsO.sub.4 continues to be preferred over KTiOPO.sub.4 for many device applications. Accordingly, processes suitable for the growth of large single crystals of CsTiOAsO.sub.4 are of interest.
Crystals of KTiOPO.sub.4 and its analogs are known to melt incongruently at temperatures from 1100.degree. C. to 1150.degree. C. Flux methods disclosed in U.S. Pat. Nos. 4,231,838 and 4,746,396 are commonly used to grow such crystals below these temperatures, typically from 850.degree. C. to 960.degree. C. Hydrothermal methods are also commonly used for the production of the crystal KTiOPO.sub.4. U.S. Pat. Nos. 4,305,778 and 3,949,323, as well as others, teach the preparation of KTiOPO.sub.4 crystals by these methods.
The orthorhombic, Pna2.sub.1 composition CsTiOAsO.sub.4 was first reported by Protas and coworkers ("Structure Cristalline de CsTiO(AsO.sub.4)", Acta Cryst., C45, 1123-1125, (1989)). Although small crystals of CsTiOAsO.sub.4 from about a few hundred microns to a few millimeters have been synthesized by the aforemention methods (see for example, Marnier et. al. , "Ferro-electric transition and melting temperatures of new compounds: CsTiOAsO.sub.4 and Cs.sub.1-x M.sub.x TiOP.sub.1-y As.sub.y O.sub.4, where M is K or Rb," J. Phys.: Condens. Matter, 1, 5509-5513 (1989)), the growth of large single crystals of CsTiOAsO.sub.4 greater than about 5.times.5.times.5 mm.sup.3 has not been reported. This is believed to be due to a heretofore unreported structural instability of CsTiOAsO.sub.4 resulting in the decomposition of the orthorhombic structure into a cubic, Cs-deficient structure at about 960.degree. C. This limits the maximum flux growth temperature to be no higher than about 960.degree. C. The aforementioned hydrothermal crystal growth methods are also considered ineffective because they generally result in the crystallization of the Cs-deficient compound having the cubic structure.